Aerodynamic Formulas questions

ThibaultHUGUET

Hi,

I'm a student in mechanical engineering and rFactor player.

I actually try to develop some tools for improve my car setup. For this, I try to do aeromaps based on HDV file.

I try some tools like CarFactory but I found too much difference between it and my measure on track for some parameters that I decided to do my own tool.

To check my calculation, I did some straight line test with data acquisition. I do constant speed test and measure wheel load and remove static load to have only aeroloads.
Also, I removed rolling resistance because this cause some load transfer too.

After I do a model of my car without aero (put 0 on almost all aero lines in HDV) and enter parameters one by one and elements by elements to understand well the parameters influence.

For many parameters, I'm in the ballpark but I have some doubt for others...

For example, I have difference for FWliftHeight effect. I use the parameter like this :

FWLift(polynomial equation) + FWliftHeight * FWCenterHeight from ground

I recalculate the height of the FW center in function of its position from the front axle and front and rear ride height.

This calculation give me less DF than Rfactor in all the case. Actually the same than Carfactory....
(I precise that all other calculation are ok and for checking this parameters, only put -1 in the first terms of FWLiftBase and FWCenter is (0,0,0).)

I haven't found other formulas on internet and can't manage to find one that work for this.

Someone already had this problem or studied this formula ?

Thanks in advance to this awesome community.
 
Hi,

welcome!

As far as i know, the parameter you mentioned normally take into account front wing ride height,as you mentioned.

This depends on FW initial location (FWCenter), ride heights, pitch and distance between FW and wheel center (lever arm).

If i take into account all these parameters, i normally get right results.

At the end, that parameter work on the real and dynamic ride height of the front wing, which depends on its location compared to car body, but also on the car body distance from ground and on its attitude.
 
silente said:
Hi,

welcome!

As far as i know, the parameter you mentioned normally take into account front wing ride height,as you mentioned.

This depends on FW initial location (FWCenter), ride heights, pitch and distance between FW and wheel center (lever arm).

If i take into account all these parameters, i normally get right results.

At the end, that parameter work on the real and dynamic ride height of the front wing, which depends on its location compared to car body, but also on the car body distance from ground and on its attitude.
Click to expand...

Thanks for your answer !

Normally I take in account all this parameters in my calculation.
In order to simplify, I put the FW center in (0,0,0) and use FWLiftParams=(-1,0,0). With this, I normally avoid problems of moving FC center because it's the FRH (measured on Data acquisition)
I use a FWLiftHeight of 5 (I know it's big but it's just to have important effect and see better the effect).

I do straight line test at more or less 151 KPH (I also take the real speed from data).

This method give me pretty accurate result because multiple test with same parameters give me same result (more or less 1N....).

Doing this with multiple FRH setup, I have multiple dynamics FRH.

You can see my result there :
View attachment 15726

Without Liftheight the DF wil be around 1770N.

Now if I divided the force found via data by Speed^2, I have my lift coefficient. If I plot it versus FRH, I had a line which equation is -5.546 x + 1.0296....
So If the equation was the one from my first post, it will be -5 * x + 1 ....

So I actually trying to explain this gap but still not found... And yes for my application, 20 N at 150 KPH is big because I need to have a very precise balance to exploit my tires...
 
are you taking into account drag Forces from wings or body and their lever arms?

Since they normally has also an application point at a certain height, that could cause something like what you describe.

Or are all your drag forces = 0?
 
I put all wings/body/diff at 0 for DF and drag except elements I mention in my previous post...

I actually can't do another test to recheck another time that I don't do mistakes in my test but I'm pretty sure that I have do it right because I have multiple test with similar results... Perhaps I will be able in one or two days...
 
I did other test and perhaps I find something.
I actually take the RH from telemetry. I do a test and it seems that it measure RH on undertray points ! And in my model, this is not the same than axle in longitudinal axis...
So this doesn't give me problems with diffuser, so it must use undertray height for the calculation and not RH at axles.
But this false my calculation of wing position (because I taken the RH from telemetry as RH axle) so this will change my FW height and my downforce measure !
And I will need to investigate for the body too because it's possible than some problem occur ...

Someone have information on where RH is taken for each elements ?
 
ThibaultHUGUET said:
I did other test and perhaps I find something.
I actually take the RH from telemetry. I do a test and it seems that it measure RH on undertray points ! And in my model, this is not the same than axle in longitudinal axis...
So this doesn't give me problems with diffuser, so it must use undertray height for the calculation and not RH at axles.
But this false my calculation of wing position (because I taken the RH from telemetry as RH axle) so this will change my FW height and my downforce measure !
And I will need to investigate for the body too because it's possible than some problem occur ...

Someone have information on where RH is taken for each elements ?
Click to expand...

yes, rFactor measures the ride heights at the undertray points. That is why they should better represent the reference plane of the car or, in anycase, the lower surface of it (on some race cars this is a wooden plate under the main underbody, for rules reasons).

So, if you want to be consistent, you should somehow be aware/careful about where your undertray points are.

The easiest way, in my experience, is to place them at the front and rear axes longitudinal position, although normally your real car underbody doesn't extend untill there. Anyway, these are the points where you normally want to identify your ride height also for some other calculations. You just need to be careful with bottoming down, in case you go this route.
 
Yes I have right now the right calculation for FWliftheight effect !
Just a small difference in diffuser repartition of lift and my model will be near to perfect ^^

Hum my car is an open wheeler.

All my PM use a datum plane as reference which is the same than my reference plane. In the real vehicle, there is a wooden plate of 5 mm below the datum plane. And this wooden plate is the reference for ride height measure.

I try to put undertray in axle position but I have too low ride height and I bottoming down in high speed (end of straight line or high speed corner). I think put the rear undertray where the rear axle is and let the front undertray where they are. This will probably permit me to put also them 5 mm below the ref plane like in reality (actually at 0) to bottoming at the right ride height. Also this will give me some mathematical gymnastic to have an aeromap that match with real car ...
 
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